The use of bulky ligand transition metal metallocene-type catalyst systems in polymerization processes to produce a diverse array of new polymers for use in a wide variety of applications and products is well known in the art. Typical bulky ligand transition metal metallocene-type compounds are generally described as containing one or more ligands capable of .eta.-5 bonding to the transition metal atom, usually, cyclopentadienyl derived ligands or moieties, in combination with a transition metal selected from Group 4, 5 or 6 or from the lanthanide and actinide series of the Periodic Table of Elements. Exemplary of the development of these and other metallocene-type catalyst compounds and catalyst systems are described in U.S. Pat. Nos. 5,017,714, 5,055,438, 5,096, 867, 5,198,401, 5,229,478, 5,264,405, 5,278,119, 5,324,800, 5,384,299, 5,408,017, 5,491,207 and 5,621,126 all of which are herein fully incorporated by reference.
Commercial polyethylene generally has a trade-off between their processability and their physical properties. Processability is the ability to economically process and shape a polymer uniformly. Processability involves such elements as thermal stability, how easily the polymer flows, melt strength, and whether or not the extrdudate is distortion free. Linear polyethylenes, known as Linear Low Density Polyethylene (LLDPE) made with traditional Ziegler-Natta catalysts, are more difficult to process than high pressure produced low density polyethylenes (LDPE) because linear polyethylenes exhibit a higher viscosity at a higher shear region which requires more motor power, produces higher extruder pressure and prone to melt fracture at the extrusion rates of LDPE's. Linear polyethylenes also have poor bubble stability compared to LDPE due to a lower melt viscosity at low shear rates and/or lower melt strength. On the other hand, however, linear polyethylenes exhibit superior physical properties as compared to LDPE's.
In order to take advantage of the superior physical and mechanical properties of linear polyethylenes, expensive antioxidants and processing aids are added to the polymer, and extrusion equipment must be modified to achieve commercial extrusion rates.
It is also common practice in the industry to add low levels of an LDPE to a linear polyethylene to increase melt strength, to increase shear sensitivity, to increase flow at a given horse power; and to reduce the tendency to melt fracture.
A second technique to improve the processability of linear polyethylenes is to broaden the molecular weight distribution by blending two or more linear polyethylenes with significantly different molecular weights, or by changing to a polymerization catalyst that produces a polymer having a broad molecular weight distribution.
Thus, in order to obtain polymer products having improved physical properties as well as easier processing polymers, the industry has focused on the physical blending of two or more polymers in the hopes that the polymer blend will exhibit the best characteristics of its component polymers. Others have looked at using two or more reactors to produce blends in situ in the reactor or use two or more catalysts to produce the desired polymer product.
There are various publications in the art discussing mixed metallocene-type catalysts. For example, U.S. Pat. No. 4,530,914 discusses a catalyst system for producing polyethylene having a broad molecular weight distribution using two different metallocene-type catalyst compounds having different propagation and termination rate constants for ethylene polymerization. U.S. Pat. No. 4,937,299 is directed to a homogeneous catalyst system of at least two metallocene-type catalyst compounds each having different reactivity ratios for use in a single reactor to produce a polymer blend. U.S. Pat. No. 5,470,811 discusses producing polymers having improved product properties using an isomeric mixture of two or more substituted metallocene-type catalyst compounds. EP-A2-0 743 327 discuss the use of meso and racemic bridged mixed metallocene-type catalysts compounds to produce ethylene polymers having improved processability. U.S. Pat. No. 5,516,848 relates to a process for producing polypropylene using a bridged mono-cyclopentadienyl heteroatom containing compound and an unbridged, bis-cyclopentadienyl containing compound. EP-B1-0 310 734 discusses the use of a mixed bridged bulky ligand hafnium and zirconium metallocene-type catalyst compounds to produce a polymer having broad molecular weight distribution. U.S. Pat. No. 5,696,045 describes using at least two different bridged, bulky ligand zirconium metallocene-type catalyst compounds to produce propylene polymers having a broad molecular weight distribution where one of the stereorigid zirconocenes has as a bulky ligand, an indenyl ligand having a substituent on the six-member ring. EP-B1-516 018 describes using two different bridged bulky ligand zirconium metallocene-type catalyst compounds to produce a broad molecular weight distribution polypropylene polymer where one of the bridged metallocenes has as the bulky ligands, indenyl ligands, that are substituted in at least the two position.
Others in the art discuss using a polymerization process in which two or more polymerization reactors are joined in series, where one catalyst is used in a first reactor to produce a first polymer that is then fed into a second reactor with the same or different catalyst, typically under different reactor conditions. In this way, the resulting polymer from a series polymerization process is a combination or blend of two different polymers. These polymer blends, typically, contain a high molecular weight and a low molecular weight component. For example, U.S. Pat. No. 5,665,818 discusses using two fluidized gas phase reactors in series using a transition metal based catalyst to form an in situ polymer blend having improved extrudability. EP-B 1-0 527 221 discusses a series reactor process using metallocene-type catalyst systems for producing bimodal molecular weight distribution polymer products. However, series or multistage reactor processes are expensive and more difficult to operate.
Thus, it would be highly advantageous to have a catalyst system capable of producing an easier processing polymer having good properties, preferably in a single polymerization process.